Methanol (MeOH) is one of the most important causes of chemical—(COD) and biological—(BOD) oxygen demand in biomass effluent streams or black liquor streams. Tightening environmental regulations make active methanol segregation and control essential. Alkaline cooking of wood chips at pulp mills forms normally 5-20 kg MeOH/t pulp, and methanol is hence found in varying amounts in all off-streams from the cooking plant, the most important being weak black liquor stream. Weak black liquor is an essential stream for producing reusable clean water because it simultaneously forms the key energy source in the pulp mill when it is concentrated by evaporation into so called firing black liquor and then burned in the recovery boiler. The modern recovery process can produce in a state of art pulp mill an excess of both heat and electricity.
The water removed from weak black liquor in the evaporation plant can contain a lot of volatile compounds like methanol, ethanol, acetone, turpentine and a number of sulphuric compounds. All these components are then partially contained in the firing black liquor but most of them are separated into secondary condensates and non-condensable off-gases.
Modern separation processes in evaporators have as a target to segregate secondary condensates so that most of the methanol is enriched in one relatively small condensate fraction (often called foul condensate), which can be purified with acceptable costs. The concentrated methanol and other volatile organic compounds can then be combusted in the recovery boiler, dedicated non-condensable gas incinerator or in the lime kiln. This, in turn, reduces the environmental impact of biomass-based methanol and also methanol accumulation when reducing fresh water consumption.
Modern pulp mills have a high degree of process integration, and black liquor evaporation is a very essential part of modern chemical-, water- and energy circulation. This has also been proven by modern pulp mills having a multi-effect evaporator train in the center of the pulp mill.
The black liquor evaporation plant typically comprises a multiple-effect evaporation plant with 3 to 7 effects (FIG. 1). Multiple-effect evaporation is in use in almost all sulphate pulp mills. The steam sequence is straight downstream. This is almost always the case. Live steam comes from the mill's low-pressure steam distribution system at a pressure of 0.35-0.45 MPa (absolute). This corresponds to a saturation temperature of 139° C.-148° C. The live steam is fed into the heating elements of the first effect (not shown in FIG. 1). Vapor generated in the liquor side of effect 1 is led through lines 15 into the heating elements of effect 2 and from there into effect 3 and so on as indicated by lines 16. Finally, vapor from the last effect 6 at a temperature of 57° C.-60° C. condenses in a surface condenser 8. In almost all cases the steam flow sequence is numbered so that it goes from effect 1 to next effect numbered as effect 2 and so on and the black liquor typically flows in the opposite direction. In FIG. 1 the heating element of the evaporators is a lamella formed of two plates attached to each other. Liquid to be evaporated is falling on the outer surface of the lamella, and a heating medium, such as vapor, flows inside the lamella. This is described in more detail in FIG. 2.
Several possibilities exist to arrange the liquor flow sequence in the evaporation plant. The optimum number of effects depends on the steam balance of the mill with boundary conditions like electricity production, electricity price, etc. Saving steam is not always economical and mill-wide cost calculations are needed to find out the best solution case by case. The application in FIG. 1 is typical for a northern pulp mill using softwood as raw material.
The concentrator effect 1 is usually divided into several subunits, which are parallel in the steam side and in series in the liquor side. For the liquor side the typical sequence is a backward- or mixed feed sequence. If the feed temperature is higher than the temperature in the last effect and the backward feed pattern is preferred, the liquor has to be flashed before feeding it to the last effect. The flashed steam from hot weak liquor is then mixed with suitable secondary steam flows giving latent heat to colder effects. The black liquor flow is indicated in FIG. 1. Weak black liquor (or other cellulose pulp waste liquor) in line 10 is fed into effect 4, where the liquor flashes. Then the liquor is passed via line 11 to effect 5, where it is further flashed. From effect 5, the liquor is passed via line 12 to effect 6 for evaporation. The liquor is further evaporated in effects 5, 4, 3, 2 as indicated by line 13. The evaporated liquor in line 14 is withdrawn from effect 2 and fed to effect 1 (not shown), in which the product liquor for burning in a recovery boiler is formed.
BOD (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) load to a waste water treatment plant can be greatly reduced by cleaning and segregating secondary condensate properly within the evaporation system. And when the quality of the secondary condensate is sufficient, all condensate can be used in the mill processes replacing fresh water intake. This remarkably reduces the environmental load.
Thanks to effective droplet separation, secondary condensate from modern evaporators contains very little salts, typically 5-10 mg Na/l for hardwood applications. All volatile components (methanol, total reduced sulfur (TRS)-compounds) from black liquor can be effectively separated from secondary condensate in the evaporator plant.
In modern evaporators secondary condensate can be fractionated inside lamellas to clean and foul condensates fractions (see FIGS. 1 and 2). The generated foul condensate is typically further processed by steam stripping. A stripping process produces stripped (clean) condensate and liquid methanol fuel. Fractionating in each effect is selected to maximize the methanol recovery while minimizing the foul condensate flow. Foul condensate amount from the evaporator plant is typically ca 15% of the total condensate amount and can give an overall methanol recovery of 70-80%. The present (duct stripping) invention can help to increase the methanol capture close to 100% when combined with evaporators that have internal condensate segregation. The condensate segregation is described below.
The vapor space inside the lamellas 20 is divided by diagonal welding seams 21 to lower 22 and upper 23 sections (FIG. 2). Most of the water vapor fed to the evaporator through line 28 is condensed in the lower section producing clean condensate 24. A smaller fraction of the vapor together with most of the methanol and TRS-compounds is condensed in the upper section 23 and collected as foul condensate 25. The foul condensate section area is 5-30% of the lamella surface, the highest in back end evaporator effects. Vent vapor is discharged from conduit 26. Liquor to be evaporated is introduced through line 29 and evaporated liquor is discharged via line 30. Vapor generated in the evaporator is taken out via line 31.
Fractionating in each effect is selected to maximize the methanol recovery while minimizing the foul condensate flow. The foul condensate amount from the evaporator stages is normally varying from 5 to 30% of the total condensate. In the evaporator shown in FIG. 2, the portion of the foul condensate is 10% of the total vapor flow in and its MeOH mass flow out stands for 80% of the total mass flow in of MeOH. The corresponding figures of the clean condensate out are 89% of total vapor mass flow in and the MeOH flow out equals 10% of total mass flow in of MeOH, and the vent vapor is 1% of total vapor mass flow in and its MeOH mass flow out stands for 10% of total mass flow in of MeOH.
Depending on condensate quality and methanol recovery requirements, the number of segregating effects and segregation area can be freely selected in evaporators. Secondary condensate fractions in a 6-effect evaporator are shown in FIG. 1. The 6-effect evaporator shown in FIG. 1 has segregation in effects 2 through 6 and in the surface condenser. Corresponding condensate flows and compositions are indicated in Table 1.
In FIG. 1 the foul condensates formed in effects 2, 3, 4, 5 are collected into a flash tank 17 and discharged through line 18. The foul condensates (FC) from the surface condenser 8 and effect 6 are also led to line 18. The clean condensates from effects 2, 3 and 4 are discharged through line 19 as secondary condensate 1 (SC1). The clean condensates from effects 5, 6 and the surface condenser 8 are discharged through line 27 as secondary condensate 2 (SC2).
Foul condensates are typically cleaned with steam in a stripping column (stripper), which is a cylindrical vessel where the liquid for stripping flows down by gravity and steam rises upward. The mass transfer process is enhanced by intermediate bottoms in the column that divide the heating and degasification of the liquid in stages. The foul condensate stripper is placed between evaporation effects 1 and 2 or 2 and 3. Secondary vapor from a previous effect is used as a heat source in the stripper. The succeeding effect has a dedicated lamella package where vapor from the top of the stripper, enriched with MeOH and TRS-compounds, is partly condensed (inside) and black liquor evaporated (outside). Non-condensed vapor is further partially condensed in a liquor preheater and the rest of stripper gases flow through a trim condenser. The MeOH content in Stripper Off Gases (SOG) is adjusted to about 30% if the gases are further processed into liquid methanol, or to approximately 30 to 50% if the gases are incinerated in a gas phase. The stripped clean condensate from the stripper bottom can be combined with clean secondary condensate.
Stripper off gases can be fired (in a dedicated non-condensable gas incinerator/lime kiln/recovery boiler) or rectified in a methanol column to liquid methanol. Liquefied methanol which can be stored and then fired in a controlled fashion has normally a water content of ca 20% and is a good fuel giving ca 15 MJ/kg.
Integrated foul condensate stripping has a major advantage over alternative systems: all energy required to clean the foul condensate can be utilized in the evaporator and stripping does not appreciably decrease the evaporator economy.
TABLE 1Condensate quality from the evaporator in FIG. 1.Flow %of totalMethanolTRScondensatemg/lmg/lRemarksSC14125<5SC244425<10Slightly odorousFC foul condensate156000<200MalodorousStripped foul15300<5Stripper MeOHcondensatepurificationefficiency = 95%
Internal and external segregation is combined in some known processes. Honkanen et al. patented a method in U.S. Pat. No. 6,797,125, where foul condensates from a previous effect are flashed in a stripper. The pure condensate fraction from evaporator D is purified in the stripper by the flashed vapors and the resulting high purity condensate fraction is taken out from the bottom of the stripper.
Olausson et al. (6) presented a following method in U.S. Pat. No. 6,258,206. In an evaporator train, where internal segregation is used, foul condensate fraction from effect 1 is fed to the upper part of the steam side of the effect 2. The clean condensate fraction from effect 3 is circulated to the lower part of effect 2 steam side. MeOH and other VOC's are concentrated into the dirty fraction from the last effect. When condensates of different purity are fed into the steam side, the least contaminated condensate is first stripped by the clean secondary vapor, which collects impurities but is still capable of purifying the more contaminated condensate in the upper part of the heat transfer section.
The most fouled condensate fraction is routed into a condensate treatment plant. A condensate treatment plant is typically integrated with the evaporator train, and it comprises a stripping column, a 2-effect evaporator, a MeOH-liquefaction column and a terpene decanter.
In a typical arrangement presented in FIG. 10, foul condensate in line 200 is first preheated in a preheater 202 with stripped condensate in line 206 from the bottom of the stripping column 204. A stripping column 204 is a tray-column, where typically secondary vapor from effect 1 in line 228 is used to remove impurities from the condensate. Contaminated stripper overhead vapor 208 is used for heating the second effect 210 in a separated heat transfer section, and condensate is flashed and recycled back to the stripping column via line 212 as a reflux. Non-condensable gases (NCG) from the second effect's separated heat transfer area can be used for preheating (in a preheater 214) liquors between effects 2 & 3. All the NCG's and flashed vapors are finally routed to a trim condenser 216, where the final condensation is done by cooling water in line 218. The resulting condensates in line 230 are mixed with the stripper reflux stream. The NCG's from the trim condenser 216 can be combusted in the recovery boiler, in the lime kiln or in a dedicated NCG incinerator. Another option is to strip NCG's (in line 220) with vapor 238 in a MeOH column 222. The overhead vapor 232 from the MeOH column is partially condensed in condenser 224 with cooling water 240 to liquid MeOH, which is taken out via line 236. Terpenes can be separated in a terpene decanter in softwood plants. The bottom stream from the MeOH column and condensed overhead vapors from the condensate stripper can be fractioned in a decanter system.